Development of Ni-doped A-site lanthanides-based perovskite-type oxide catalysts for CO2 methanation by auto-combustion method

Engineering the interfacial interaction between the active metal element and support material is a promising strategy for improving the performance of catalysts toward CO2 methanation. Herein, the Ni-doped rare-earth metal-based A-site substituted perovskite-type oxide catalysts (Ni/AMnO3; A = Sm, La, Nd, Ce, Pr) were synthesized by auto-combustion method, thoroughly characterized, and evaluated for CO2 methanation reaction. The XRD analysis confirmed the perovskite structure and the formation of nano-size particles with crystallite sizes ranging from 18 to 47 nm. The Ni/CeMnO3 catalyst exhibited a higher CO2 conversion rate of 6.6 × 10−5 molCO2 gcat−1 s−1 and high selectivity towards CH4 formation due to the surface composition of the active sites and capability to activate CO2 molecules under redox property adopted associative and dissociative mechanisms. The higher activity of the catalyst could be attributed to the strong metal–support interface, available active sites, surface basicity, and higher surface area. XRD analysis of spent catalysts showed enlarged crystallite size, indicating particle aggregation during the reaction; nevertheless, the cerium-containing catalyst displayed the least increase, demonstrating resilience, structural stability, and potential for CO2 methanation reaction.


Introduction
Fossil fuel utilization to fulll domestic and industrial energy demands has triggered the emission of greenhouse gases, mainly carbon dioxide (CO 2 ).The CO 2 emissions have aggravated global warming, badly affecting life on Earth.The scien-tic community is committed to addressing the global warming problem by reutilizing atmospheric CO 2 through carbon capture and utilization (CCU) as well as carbon capture and storage (CCS) technologies. 1,2Among the adopted routes, catalytic CO 2 conversion has received signicant attention to produce renewable fuels and commodity chemicals. 3[6] CO 2 + 4H 2 4 CH 4 + 2H 2 O, DH 298 K = −164.7 kJ mol −1 (1) CO 2 hydrogenation is thermodynamically an exothermic reaction favored under mild conditions like low reaction temperature and atmospheric pressure. 7In addition, high temperature affects the selectivity of the product.It increases the carbon monoxide (CO) production through various side reactions i.e. reverse water gas shi reaction (RWGS) (CO 2 + H 2 4 CO + H 2 O), Reverse-Boudouard (RB) reaction (C + CO 2 4 2CO).The RB and other side reactions can result in the deactivation of the catalyst due to carbon deposition.[10] According to the literature, Ni-supported catalysts have extraordinary merits due to their lower cost, higher selectivity, and better stability.5][16] Regarding Ni-based PTOs, studies have reported that the rare earth elements (La, Ce, Sm, etc.) enhance the CO 2 activation and act as a modier to tune the basicity, metal dispersion, and metal-support interaction.Adding La 2 O 3 intensies the CO 2 chemisorption, resulting in H 2 dissociation and CO 2 activation at a lower temperature. 17Similarly, cerium oxide (CeO 2 ), samarium oxide (Sm 2 O 3 ), and praseodymium oxide (Pr n O 2n−n ) have been employed as catalytic promotors to tune the metal-support interaction.Further, promotors enhance the surface basicity due to the interaction of their oxygen vacancies with the oxygen atom of the CO 2 molecule, weakening the C]O bond strength 18 and, subsequently, enhancing the catalytic activity of Ni-based systems towards CO 2 hydrogenation.Likewise, it has been reported that manganese (Mn) substitution at the B sites enhances the oxygen exchange capacities within the PTOs structures, favoring the optimal catalytic performance as reported for Ni/YMn 1−x Al x O 3 perovskite catalysts. 6Perovskites ABO 3 with substituted Mn species at B sites might create the defect chemistry within PTOs structure, thereby promoting the number of non-stoichiometric vacancies and enhancing the lattice oxygen. 19Keeping the xed B position, variation at the A site of perovskite material leads to improved surface oxygen concentration, specic surface area and lowers the reduction temperature of the catalysts. 20To our knowledge, introducing lanthanide metals at A-sites while keeping Mn at B-sites in PTOs has rarely been explored for CO 2 hydrogenation.In view of the above, the present work reports different perovskite-type supports (A x Mn y O 3 , A = Sm, La, Nd, Ce, Pr with x, y = 1) prepared by A-site substitution of rare earth metals through one-pot auto-combustion method followed by Ni loading via impregnation method.The prepared catalysts were investigated for hydrogenation of CO 2 into CH 4 .Thus, the behavior and potential of 10% by wt Ni/AMnO 3 perovskites series of catalysts were evaluated.The higher performance exhibited by the Ni/CeMnO 3 catalyst showed the potential of PTOs for CO 2 methanation.Perovskite oxides A x Mn y O 3 (x, y = 1, rare earth elements denoted as A, A = Sm, La, Nd, Ce, Pr) were prepared by the onepot auto-combustion method.Initially, the stoichiometric amounts of each rare earth metals (A = Sm, La, Nd, Ce, Pr) were mixed with Mn(NO 3 ) 2 $4H 2 O separately as a precursor and dissolved in deionized water and stirred for 60 min to obtain a homogeneous mixture.Then, the citric acid (CA, VWR) as a fuel was dissolved with a molar ratio CA : Mn : A = 2 : 1 : 1 and stirred for 30 min, followed by the addition of ethylene glycol (EG, Merck) to the above mixture with a molar ratio of EG : CA = 1.2 : 1 and stirred for 2 h at 100 °C.Subsequently, the solution temperature was gradually increased to 350 °C with a xed time interval (20 min) to evaporate the solvents until a gel-like material was obtained.Upon further heating, the gel was transformed into a foam-like structure, which self-ignited and turned into akes.The akes were further crushed to raw powder, dried at 120 °C, and calcined in air at 900 °C for 4 h with a heating rate of 10 °C min −1 . 21The obtained perovskitelike supports SmMnO 3 , LaMnO 3 , NdMnO 3 , CeMnO 3 , and PrMnO 3 were referred to as SM, LM, NM, CM, and PM.

Experimental section
To obtain Ni-doped PTOs, 10% by wt Ni was impregnated over the prepared support using the wetness impregnation method. 16The Ni precursor dissolved in de-ionized water was added to the perovskite supports and vigorously stirred on a hot plate at 100 °C until the solvent was evaporated entirely.The obtained materials were dried overnight at 120 °C, calcined at 500 °C (10 °C min −1 ) for 4 h in air, ground, and sieved accordingly.The synthesized catalysts were named NSM, NLM, NNM, NCM, and NPM, respectively.

Characterization
The chemical composition of the samples was measured by ICP-AES and ICP-SFMS.The samples were digested in nitric acid and analyzed in an Agilent 4200 MP-AES for the measurements.The crystallinity of the PTOs was analyzed by X-ray diffraction using the Cu-based anode (Cu-Ka radiation, l = 1.5418Å) with a Bruker D2 Phaser Instrument.The diffractograms were acquired in the 2q range of 10 to 80°with step size of 0.02°and step time of 0.35 s, respectively.The crystallite sizes were estimated using the Scherrer equation (eqn (2)), where D is the mean particle size, K is a dimensionless shape factor that typically accounts for the shape of the particle and is generally taken to have the value 0.94-0.97,l is the X-ray wavelength, b is the line broadening at half of the maximum intensity (FWHM), and q is the Bragg angle in radian units.
The H 2 -TPR and CO 2 -TPD experiments were carried out in calibrated ChemBET Pulsar (AntonPar) connected with a thermal conductivity detector (TCD).Typically, 50 mg of sample was loaded in a quartz U-shaped reactor, and the reducibility prole was recorded using 10% H 2 /N 2 up to 900 °C with a heating rate of 10 °C min −1 .For CO 2 -TPD, 50 mg of each sample was pre-reduced in 10%H 2 /N 2 gas at 600 °C using a 10 °C min −1 heating rate and cooled down under the He atmosphere (30 ml min −1 ).The pre-reduced samples were exposed to 10% CO 2 /N 2 for 45 min.Then, the physisorbed CO 2 species were ushed out using He for 30 min.The chemisorbed CO 2 -TPD prole was recorded continuously in N 2 ow using a heating rate of 20 °C min −1 up to 600 °C.Fourier Transform Infrared Transmission (FT-IR) spectra were studied using the BRUKER ALPHA PLATINUM-ATR spectrometer.For experiments, a 10 mg sieved powder sample (100-200 mm) was loaded on ATR crystal to perform testing using OPUS soware in the range of 400 to 4000 cm −1 .The surface area and pore size distribution were evaluated by N 2 -physisorption measurements using Gemini VII 2390, Micromeritics, Norcross, USA) at 77 K. Before the analysis, the samples were degassed for 2 h at 350 °C under vacuum.The surface morphology of the samples was analyzed by SEM equipped with EDS using Oxford Instrument (JSM-IT300LV, JEOL GmbH, Germany).Furthermore, the cyclability of the in situ pre-reduced supports and the catalysts were tested through a series of oxidative and reductive cycles using METTLER Toledo TGA/SDTA 851e Thermogravimetric Instrument at 400 °© C. Typically, 50-60 mg samples weighed in 40 ml alumina crucibles were reduced in 4% (H 2 /N 2 ) mixture followed by oxidation (using 76 ml per min N 2 , 4 ml per min CO 2 , 20 ml per min N 2 -as carrier) and reduction (using 76 ml per min N 2 , 4 ml per min H 2 , 20 ml per min N 2 -as carrier).The reversible redox capacities of the catalysts were estimated by weight variations (wt%) under optimized reaction conditions, which was linked to their O-exchange capabilities.The surface elemental composition of the catalyst was analyzed by X-ray photoelectron spectroscopy (XPS).In addition, coke formation during the CO 2 methanation reaction over catalysts was analyzed using MET-TLER Toledo TGA/SDTA 851e Thermogravimetric Instrument.A 50 mg of spent catalyst was lled in a 40 ml alumina crucible and heated from ambient temperature to 900 °C with a ramping rate of 15 °C min −1 under air (20% O 2 /80% N 2 ).

Catalytic activity
The catalytic performance of the catalysts was evaluated using a tubular reactor (inner diameter = 0.9 cm) connected with a digital interface Micromeritics PID Eng & Tech controller and ABB AO2020 analyzers, as shown in Fig. 1.The ABB analyzing set-up is equipped with Caldos 25 and Uras 26 detection housing units for continuous measurement of gas composition at the reactor outlet.
The reaction was carried out by loading 100 mg of catalyst (100-200 mm size) diluted with SiC up to 0.5 cm 3 in the reactor with a space velocity of 12 000 h −1 .The catalysts were in situ reduced at 600 °C for 30 min under a 20% H 2 /N 2 gas mixture.Then, the reactor was cooled down to 200 °C, and the reaction was carried out between 200-600 °C at atmospheric pressure.Feed gas CO 2 /H 2 with the ratio of 1/4 (balanced with N 2 ) was introduced, and product gas composition was monitored with ABB analyzers.The exhibited catalytic activity reects the average conversion and selectivity values over a stable temperature.Furthermore, the effect of CO 2 partial pressure and ow rate was also investigated.The stability test was recorded at a constant temperature of 400 °C for 50 h under similar reaction conditions.The conversion and selectivities were calculated using the following (eqn (3)-( 5)).

Catalyst composition
The chemical composition of the calcined supports and catalysts determined by ICP-AES are presented in Table 1.The Cecontaining samples were analyzed using ICP-SFMS due to the limitation of the ICP-AES instrument.The elemental compositions of all the samples were close to the calculated compositions.The evaluated Ni contents were found in an acceptable range for all samples, as 10 wt% Ni was loaded into each synthesized catalyst.Assuming no impurities, the rest of the contribution in each experimental formula justies the oxygen composition.Perovskite structure can be predicted by the GoldSchmidt tolerance factor (t). Ideally, the perovskite has a tolerance factor of 1 for a cubic symmetric structure.For nonsymmetric structures, the tolerance factor values are in the range of 0.75 < t < 1, which can be calculated by the following (eqn ( 6)).

Tolerance factor; ðTFÞ ¼ ðr
where r A , r B and r O represent the ionic radii of A, B, and O in the ABO 3 perovskite structure.Table 1 lists the calculated values of the tolerance factor for perovskite supports, which are in good agreement with the non-symmetric perovskite structure.1][22] It has been reported that the small amount of Ce and Mn elements might not be well incorporated into the perovskite structure. 23he possible reason for the presence of CeO 2 and Mn x O y species may be charge imbalance within the perovskite structure owing to different valence states of Ce and Mn at elevated temperatures.As a result, a minor quantity of Ce and Mn oxides appeared along with the perovskite oxide structure.2).In fact, the lattice parameters started rearranging within the structure of perovskite, which could cause a decrease in lattice parameters of spent catalysts, leading to the contraction of lattice and peak angles shied to slightly higher 2q values.It is also reected that structural tilting brings lattice changes in the existing perovskite crystal structure that alter the lattice parameters by comprehensive strain and cause the shrinkage of the respective perovskite structure.Resultantly, the main perovskite peaks shied at a higher 2q range (2q = 35.78°). 24,25dditionally, the XRD analysis of spent catalysts assures the structure stability of re-arranged perovskite phases.Minor peaks might disappear in the case of NLM and NPM catalysts, likely due to their characteristic instability. 18s a reference, the Ni (JCDPS 087-0712) and NiO (JCDPS 047-1049) positions are marked with vertical orange and green bars.The absence of NiO peaks in calcined catalysts accentuates the well-dispersed Ni particles. 26Further, the crystallite size calculated from the diffractogram of spent catalysts increased, which shows possible sintering of Ni during the reaction and has been reported in the literature as well.The diffractograms of reduced catalysts do not show diffraction peaks from reduced Ni metal (Fig. 2(c)).Thus, the disappearance of diffraction peaks at attributed positions justies the uniform dispersion and incorporation of Ni into the lattice of perovskite.

Basic and redox properties
Fig. 3(a) displayed the CO 2 -TPD proles of the catalysts recorded as a function of temperature and the quantication of CO 2 desorbed, which is listed in Table 2.The CO 2 desorption proles show weak (<300 °C), moderate (300-500 °C), and strong (>500 °C) basic sites. 27The CO 2 adsorption and activation are associated with the strength of the species in the range of weak and moderate basic sites. 13The CO 2 is adsorbed on the catalyst surface as monodentate over weak basic sites, whereas at medium and strong basic sites, CO 2 is strongly attached to surface oxides in the form of bidentate. 28All the catalysts exhibited weak and moderate surface basic sites in a comparable range.The surface of the catalyst involved heterogeneous types of weak and strong basic sites.The CO 2 adsorption sites with different strengths lead to inequivalent amounts of chemisorbed CO 2 species.The reduced conditions favor the CO 2 adsorption to cationic sites, which leads to weak adsorption of CO 2 molecules to the catalyst surface. 29The NCM catalyst displayed the highest CO 2 desorption capacity in the  given temperature range, reecting greater basic sites that would favor the CO 2 activation mechanism.The decrease in CO 2 desorption follows the order of NCM > NPM > NLM > NSM > NNM.Fig. 3(b) shows the H 2 -TPR test normalized reduction proles by the calcined support (light-pattern) paired with respective catalysts (bold-pattern).Perovskites displayed two reduction regions at 290-560 °C and 580-850 °C, respectively.1][32] Here, the consumption of H 2 species indicates the presence of oxygen vacancies in perovskite oxides.However, the reduction peak of Mn to Mn 0 was not found. 33The Ni-doped PTOs showed strong reduction peaks for NSM, NLM, NNM, and NPM between 600-850 °C, along with two minor peaks around 290-560 °C.Minor reduction peaks below 600 °C depicted the fewer oxygen species available at the catalyst surface.
Interestingly, the NCM catalyst exhibited a notable shi in peak to a lower temperature with a side dome that underscores the presence of Ni particles, which support the earlier activation of the H 2 to H.The low temperature reduction peaks (208 and 331 °C) are attributed to the reduction of surface oxygen species, which further contribute to forming oxygen vacancies in the Ni-PTOs interface and structure. 34Further, a slight shi in the reduction peak of Ni-supported LM and PM perovskites at lower temperature regions were also associated with the presence of Ni particles, promoting the lattice species reduction.In the same scenario, the NSM catalyst displayed a slightly delayed reduction, but no change was observed for the NCM catalyst at the higher temperature reduction region.Overall, the NCM catalyst showed improved redox properties with reduction peaks at a lower temperature.The quantication of the total H 2 uptake is listed in Table 2, including the contribution of the higher redox peak at elevated temperature (>600 °C) depicted by Ni-doped SM, LM, NM, and PM except for CM, which might be attributed to the bulk reduction of O species.

FTIR spectra
Fig. 4(a and b) show the FTIR spectra of calcined perovskite supports and Ni-doped PTOs.All the samples showed strong bending characteristics around 590 cm −1 , which corresponded to Mn-O-Mn deformation associated with the octahedron MnO 6 .The CM support displayed another bending around 488 cm −1 attributed to Mn-O stretching.A small vibration around 830-690 cm −1 indicates the presence of Sm-O, La-O, Nd-O, Ce-O, and Pr-O bonding.[37][38]

N 2 adsorption-desorption isotherms
The surface properties such as surface area, pore size, and pore volume are crucially linked to the active sites, heat, and mass transfer mechanisms.The N 2 adsorption and desorption isotherms were performed to understand the porosity and textural properties of the catalysts, as shown in Fig. 5(a and b).The isotherms showed the type IV curve with a small H3-like hysteresis as classied by the IUPAC. 39The inset plots showed mainly micropores (<2 nm) with a small pore distribution of mesopores (>2 nm) for synthesized and spent catalysts.The H3 hysteresis resembles slit-like pore geometry.At relatively higher pressures above (P/P 0 > 0.4), the adsorption isotherms indicated the multiple layers deposition of N 2 molecules.The textural properties evaluated from nitrogen sorption isotherms are listed in Table 3.The S BET area decreases in the order: NNM < NLM < NPM < NCM < NSM.The NSM catalyst showed the largest surface area (23.1 m 2 g −1 ), while the NNM catalyst showed the lowest surface area among the calcined catalysts.The surface area of the NCM was close to the NSM catalyst.
According to the pore size displayed in the inset picture, NCM catalysts have larger pore sizes compared to other catalysts, which would be helpful in catalytic activity.In addition, the surface area of the spent catalysts was drastically decreased, and the trend was similar in all the catalysts.The decreased surface area could probably have resulted from the sintering of Ni particles and coke formation during the CO 2 methanation reaction.An increase in the crystallite size of the spent catalysts

SEM-EDS analysis
Fig. 6 displayed the morphology and elemental composition of perovskite supports and Ni-doped PTOs investigated by the SEM-EDS.SEM micrographs revealed that perovskite supports and catalysts have a porous structure consisting of tiny, nonuniform, and irregular shape boundaries.The creation of a porous network in the solid phase for all samples linked to the solution combustion method.The evolution of gaseous products is governed by the thermal effects that tend to escape the solid surfaces, forming various pore size distributions.The porous morphology and microstructural characteristics of materials are strongly linked to gas phase catalytic activities.
Both perovskite support and catalyst showed a porous structure with a certain pore width and diameter, as evaluated through N 2 adsorption and deportation isotherms.The EDS analysis provided the elemental information of the prepared supports and catalysts, as presented in Table 4.The chemical compositions observed by EDS revealed the surface features of the catalysts.The elemental composition from EDS and ICP were quite close, with slight variations for both supports and catalysts.

Cyclic reversibility and structural stability
The cyclic reversibility of materials plays a crucial role in longterm application for catalytic reactions.Fig. 7(a and b) present the weight variation response for the samples when they were exposed to CO 2 and H 2 streams, alternatively for 10 cycles at 400 °C.The weight variation is attributed to the oxygen exchange capabilities of the materials.Except for 1 st cycle (where the sample was exposed aer pre-reduction at 600 °C under the H 2 atmosphere), samples showed a constant exchange capacity under redox cycles.The Ce-based magnetite perovskite displayed better oxygen exchange capacities in all redox cycles, as shown in Fig. 7(a), in comparison to different synthesized supports.Interestingly, nickel doping enhanced the oxygen exchange capacities of the La, Pr, and Nd magnate-based catalysts, whereas Sm and Ce-based Ni-doped perovskite displayed relatively constant oxygen vacancies up to 10 redox cycles.The Ni-doping promotes oxygen exchange capabilities by disturbing the charge neutrality of the lattice structure.The non-symmetric perovskite structural defects chemistry favored the oxygen mobility from surface to lattice and lattice to surface only by the oxygen vacancies . 40The cations and morphology of the samples at the A and B sites affect the oxygen capacities by regulating the valence state of Mn ions.The electronegativity principle explained the formation of Mn mixed-valence ions by excess oxygen and substituting lower-valence cations at the A or B sites. 41Table 3 Textural properties of the catalysts S BET,fresh (m 2 g −1 ) S BET,spent a (m 2 g −1 ) V pore,fresh a × 10 −3 (cm 3 g −1 ) V pore,spent a × 10 −3 (cm In addition, Ni doping creates the additional Mn 4+ ions that promote oxygen capacities by adjusting the structural defects in Ni-doped LaMnO 3 .On the contrary, the disappearance of defect chemistry in perovskite structures restricts the migration of oxygen species, resulting in a reduction of lattice oxygen activities. 42Additionally, catalytic activity depends on available oxygen vacancies and the Ni-metal interaction, metal dispersion, reduced Ni-sites for H 2 dissociation, the valence state of Asite element, surface area, and affinity to CO 2 as well as H 2 O species. 43Fig. 7(c) shows the reversibility factor of the Ni-doped perovskites corresponding to the redox cycles.It indicates that the prepared Ni-doped perovskites retained the oxygen exchange capacities in each cycle except the rst cycle. 44The structural stability holds the key in assessing the catalyst performance, which is directly affected by harsh reaction conditions.The XRD analysis of spent catalysts displayed a stable structure indexed with pure perovskite phases, as shown in Fig. 2(b).Also, the stable perovskite structure agreed to attain the constant reversibility of Ni-PTOs over repeated cycles, as shown in Fig. 7(c).Here, the reversibility factor is estimated based on oxygen exchange capacities in each oxidative and reductive cycle for all spent catalysts.The unity value for all the samples, irrespective of 1 st cycle, indicated the constant oxygen diffusion related to homogenous porosity and pore size distribution over the surface of the catalyst.

XPS analysis
Fig. 8 shows the XPS analysis of the NCM catalyst.The Ni deconvoluted spectra in Fig. 8(a) showed two peaks at 855.1 eV and 856.4 eV, corresponding to the NiO present on the surface and in the lattice of the perovskite, respectively. 16Fig.8(b) displayed Mn deconvolution spectra showed main peaks attributable to the spin orbits for Mn 2p 3/2 and Mn 2p 1/2 at 642.3 and 640.5 eV, revealing the prevalence of Mn 3+ ion. 45These energy values represented various mixed manganese oxides, perovskite, and spinel structures, which are normally associated with Mn 4+ and Mn 3+/ Mn 2+ type species. 46Fig. 8(c) indicated Ce 3d spectra, exhibiting a typical peak at 882.8 eV, corresponding to the Ce 4+ , proving an occupancy of the Ce 4+ oxidation state in the CeMnO 3 perovskite catalyst. 47,48The O 1s spectra exhibited two peaks at 530.2 eV and 531.8 eV, respectively as shown in Fig. 8(d).These oxygen peaks could be related to the lattice oxygen species of perovskite and surface-adsorbed oxygen-decient vacancies of CeMnO 3 perovskite, thus conrming the typical perovskite behavior.The presence of Ce in the perovskite  Paper RSC Advances could increase the mobility of lattice oxygen and transform it into surface oxygen, which is believed to be useful for the oxidation of carbon formed during the reaction. 49

Catalytic activity test
The catalytic activities of the Ni-doped PTOs were evaluated for CO 2 methanation under the following reaction conditions: H 2 / CO 2 = 4, GHSV = 12 000 h −1 , 200-550 °C, and 1 atm.Fig. 9(a) exhibits the sinusoidal s-curve type CO 2 conversion proles of all the catalysts, which are kinetically controlled at lower reaction conditions and thermodynamically limited at higher temperature ranges.The exothermic nature of the CO 2 methanation limits the reaction at elevated temperatures, which adversely drops CO 2 conversion and CH 4 selectivity.Among all, the NCM catalyst achieved the highest CO 2 conversion ca.56%, with a reaction rate of ca.6.6 × 10 −5 mol CO 2 g cat −1 s −1 at 400 °C, whereas the NLM catalyst showed the lowest catalytic activity with CO 2 conversion ca.23% with a rate of 2.75 × 10 −5 mol CO 2 g cat −1 s −1 at 400 °C.Performance analysis of the catalysts follows descending trends: NCM > NSM > NNM > NPM > NLM.Superior basicity and redox properties showed by the NCM catalyst were instrumental in activating CO 2 molecules and resultantly enhancing catalytic activity.The dominant catalytic activity of the Ce-based oxides among the presented lanthanide metals has been reported, and results were attributed to the crucial factors that support the higher CO 2 conversion and CH 4 selectivity. 43 Interestingly, catalysts revealed varying selectivity prole data, where NCM catalyst showed the highest CH 4 selectivity than CO.The CO selectivity slightly increased at higher temperatures.It shows that an increase in temperature gradually decreased the CH 4 selectivity.A possible reason may be triggering the reverse water-gas shi reaction (RWGS) with the rise in temperature, which supports CO production, thereby decreasing the CH 4 selectivity. 50,51mong Ni-based perovskite catalysts, NCM catalyst depicted ca.2.4 times higher CO 2 conversion (%) values than NLM catalyst at 400 °C.The better conversion performance is crucially attributed to multiple supporting factors that promote the reaction mechanism.The ongoing catalytic process on the catalyst surface is positively associated with surface area, and heat and mass transfer distribution inside the catalyst is strongly inuenced by the pore size characteristics of the catalysts.The homogeneous and uniform pore size mainly contributes to the CO 2 conversion, providing a chance to carry a chemical reaction with better conversion rates at the outer surface but also inside the pores of the catalysts.The BET analysis linked with Fig. 5, and SEM micrographs (Fig. 6) revealed that higher performance of the NCM catalyst attributed to the better surface area, uniformly distributed microchannel, and homogenous orientation of the Ni-doped PTOs structure.The structural stability also served a pivotal role in the sustainable conversion rate.Also, the XRD studies of the spentcatalysts samples showed Ni-perovskite structural stability (Fig. 2(b)).surface for reactants and subsequently decreased the CO 2 conversion.There was also increased crystallite size in the XRD pattern of spent catalysts, which shows the sintering was happening and might have led to a reduction in the CO 2 conversion efficiency of the catalysts.The sintering of Ni-based catalysts is not unusual and has been a constant challenge in the development of stable catalysts.Sintering leads to aggregation of Ni nanoparticles and reduces the active surface area of the catalysts, thus leading to decreased catalytic activity.The surface and pore volumes of the spent catalysts were substantially lower than the synthesized catalyst, which reaffirms the sintering and coke formation.The coke formation can lead to blockage of the pores and also wrapping of the Ni nanoparticles, rendering them inaccessible to the reacting gases.The reduction in the pore volume shows the coke deposition.Although Ce-based catalysts are good at consuming the coke and show better resilience, but decrease in activity cannot be fully avoided because both the sintering and coking are happening with time and can multiply the negative impact on the mitigating catalytic performance.Furthermore, the effect of the CO 2 partial pressure was evaluated while keeping the other reaction parameters constant, as shown in Fig. 9(d).The highest CO 2 conversion was exhibited by the NCM catalyst at 15% CO 2 injection at a ratio of H 2 /CO 2 = 4 (balanced with N 2 ).The CO 2 valorization is limited due to the chemical inertness of carbon dioxide molecules.Studies presented that CO 2 concentration is associated thermodynamically with an optimal range window to achieve the equilibrium CO 2 conversion.The strongly inert nature of the C]O double bond hindered the reaction mechanism at lower and higher CO 2 partial concentrations away from moderate CO 2 limits. 52Moreover, for the catalyst series, experiments were also conducted to study the effect of different feed ow rates on % CO 2 conversion at 400 °C, as shown in Fig. 9(e).It revealed that comparatively, the lower feed ow rate favored the better CO 2 conversion.
Table 5 compares catalytic activity for recently published Ni perovskites catalysts regarding CO 2 methanation reaction.Different preparation strategies and metal loading and reaction conditions were investigated.Compared to the listed Ni-based perovskites, the NCM catalyst prepared by the solution combustion method in this work exhibited competitive performance, attaining a remarkable CO 2 conversion into methane.In CO 2 methanation, the redox reaction system improved with proper A/B site cationic substitutions with variable valence metals, i.e. (Ce with Mn and Cu as varying valence metals) in the perovskite oxide structure.Here, the A-site cations Ce 3+ /Ce 4+ with variable valence metal (or Mn 2+,3+,4+, etc.) reversibly shuttled between PTOs and cationic oxides, which favors the perovskite support nature and resultantly improved the catalytic activity. 53The synthesis approach and the cationic substitutions helped to tune the physiochemical properties and promote the catalytic activity.In fact, the catalyst fabrication methods envisaged to attain better characterization results such as hydrogen consumption, CO 2 uptake capacity, high surface area, and uniform dispersion.These factors support the promoted catalytic performance while activating the reaction  Catalytic CO 2 methanation can proceed through two reaction pathways, such as associative and dissociative mechanisms.In the dissociative mechanism, the CO 2 molecules chemisorb and dissociate, constituting the carbonyl intermediates.For the associative mechanism, CO 2 is molecularly chemisorbed, and the oxygen of CO 2 reacts with hydrogen in sequential steps.Nevertheless, the dominant reaction pathway depends on the catalyst's nature, structure, and operating conditions. 50For the redox-type perovskite materials, higher reaction rates are related to (i) bifunctional surface where CO 2 molecules are activated over available O-vacancies provided by support and adsorbed H 2 dissociates over active Ni metal and (ii) the generation of monodentate carbonate intermediates, characterized by faster decomposition rates and associated to improved rates for methane formation. 6ithin the catalyst series, the superior catalytic performance displayed by the Ni-CeMnO 3 could be related to the promoted generation and H-assisted decomposition of reaction intermediates, resulting in efficient metal-support synergy.Lowtemperature activity of Ni/CeMnO 3 towards CO 2 methanation could proceed through a formate route mechanism. 57Both the oxide support and metallic Ni sites can interplay in the CO 2 hydrogenation process.For the Ce-containing catalysts, bidentate carbonate species originated the formate intermediates, which are adsorbed on the oxygen vacancies.The Ce ions in the lattice structure are in intimate contact with the active metal Ni 0 , available at the metal-support interface.Therefore, CO 2 adsorption over redox supports having Ce ions in the catalysts facilitating the formation of bidentate carbonate species and stabilizing the formate intermediates over the surface.The Ce ions concentration within redox support escalates the transformation of formate intermediates into methane formation over O-vacancies.Therefore, CO 2 hydrogenation over Ni-CeMnO 3 could follow the associative mechanism for CO 2 methanation by formate route.Thus, the better oxide surfaces, smaller crystal sizes, and distorted structures attained in the Ni-CeMnO 3 sample, reected in the promoted redox behavior, should encourage the activation of CO 2 molecules and dispersed Ni particles favor the hydrogenation of reaction intermediates via H 2 -spillover effects.

TGA analysis of spent catalysts
Fig. 11 shows the TGA proles of the spent catalysts under a gas mixture of 20% O 2 balanced with 80% nitrogen to estimate the carbon deposition on the catalysts.In all the spent catalysts, no sign of reverse oxidation of each sample was observed below 400 °C.Although, the spent Ni/perovskite catalysts exhibited the typical weight increase phenomenon in the range of 253 °C to 796 °C, which corresponds with the oxidation of metallic Ni on the surface of catalysts. 58Subsequently, all curves of spent catalysts depicted a slight weight loss at an elevated temperature of 800 °C to 900 °C, which might be attributed to carbon oxidation.The obtained results showed that the spent NCMactivity sample has less decline in weight loss compared to the rest of the used catalysts.However, there was no signicant weight loss behavior to explore the carbon deposition amount for these best-performing samples for CO 2 methanation activity along with the used long-term NCM-stability catalyst.Although difficult to quantify, the decrease in mass loss at high temperatures shows that there was slight coke formation.This might have contributed toward the reduced pore volume and surface area of the catalysts, thus leading to decreased catalytic performance with time.

Conclusions
In this study, Ni-based (AMnO 3 , A = Sm, La, Nd, Ce, Pr) perovskites catalysts were synthesized via the auto-combustion method, characterized, and evaluated for CO 2 methanation.The Ni/CeMnO 3 (NCM) catalyst exhibited a higher CO 2 conversion of ca.56% at 400 °C and GHSV of 12 000 h −1 .The incorporation of Ni species into the lattice provided additional surface area within the porous structure and promoted the reaction mechanism with higher active sites, as reected by the performance of the NCM catalyst.On the other hand, Ni-based La perovskite catalyst exhibited the lowest activity (ca.23% conversion) due to strongly chemisorbed species on the surface, resulting in fewer active sites for reaction.The NCM catalyst demonstrated remarkable resistance to coking and exhibited the least amount of crystallite aggregation, as evidenced by the analysis of the spent catalyst.Moreover, Ce could promote the redox process at the metal-support interface.Thus, highly reducible perovskite support established the adsorption and activation mechanism, and the extent of metal-metal synergy signicantly improved the catalytic activity and suppressed the coke formation during CO 2 methanation.The encouraging CO 2 methanation results displayed by the Ni-based CM-perovskite catalyst highlighted an innovative prospect toward the adaptability of perovskite-supported catalysts in process engineering applications.

Fig. 1
Fig.1The schematic diagram for the experimental setup.

14 - 17 3. 2 XRD
XRD diffractograms displayed by the support materials (light prole) and catalysts (bold prole) are shown in Fig.2(a).The peaks with the diamond symbol are attributed to the characteristic peaks of the perovskite crystal structure, which conrmed the successful formation of the perovskite phases.All the samples demonstrated rare-earth manganates as the main phase.Characteristics diffraction peaks of rare-earth oxides disappeared for catalysts, except CeMnO 3, which showed additional peaks of CeO 2 (2q = 28.65°,76.83°) and Mn 3 O 4 (2q =

Fig. 2 (
Fig.2(b) displays the XRD diffractograms of the spent catalysts.The slight peak shi on higher 2q values was attributed to higher crystallite size reected by spent catalysts (ref.Table 2).In fact, the lattice parameters started rearranging within the structure of perovskite, which could cause a decrease in lattice parameters of spent catalysts, leading to the contraction of lattice and peak angles shied to slightly higher 2q values.It is also reected that structural tilting brings lattice changes in the existing perovskite crystal structure that alter the lattice parameters by comprehensive strain and cause the shrinkage of the respective perovskite structure.Resultantly, the main perovskite peaks shied at a higher 2q range (2q = 35.78°).24,25Additionally, the XRD analysis of spent catalysts assures the structure stability of re-arranged perovskite phases.Minor peaks might disappear in the case of NLM and NPM catalysts, likely due to their characteristic instability.18As a reference, the Ni (JCDPS 087-0712) and NiO (JCDPS 047-1049) positions are marked with vertical orange and green bars.The absence of NiO peaks in calcined catalysts accentuates the well-dispersed Ni particles.26Further, the crystallite size calculated from the diffractogram of spent catalysts increased, which shows possible sintering of Ni during the reaction and has been reported in the literature as well.The diffractograms of reduced catalysts do not show diffraction peaks from reduced Ni metal (Fig.2(c)).Thus, the disappearance of diffraction peaks at attributed positions justies the uniform dispersion and incorporation of Ni into the lattice of perovskite.

Fig. 3
Fig. 3 (a) CO 2 -TPD profiles of the pre-reduced catalysts and (b) H 2 -TPR of supports and catalysts.

Fig. 9 (
c) shows the stability test for the NCM catalyst carried out at 400 °C with high CH 4 selectivity.The catalyst demonstrated a slight decline in conversion and CH 4 selectivity during 50 h.The gradual drop in CO 2 conversion is associated with specic agglomeration and deactivation due to the coking phenomena of Ni-based catalysts.The SEM micrograph of the spent NCM reected the sign of agglomeration and distorted structure, as shown in Fig.9(f), which reduced the available

Fig. 10
Fig. 10 Schematic mechanism of CO 2 methanation showing the various possible pathways.

Table 1
Elemental composition of the samples (metals) a From GoldSchmidt tolerance factor.

Table 2
Structural characteristics and CO 2 and H 2 quantification values of the synthesized materials a Crystal size estimated via Scherrer's equation.CO 2 and H 2 amounts were determined using pre-dened calibration values for the experimental concentration of both gases.
a CS:

Table 5
Comparison of the performance of Ni-based perovskite catalysts 54,55ules, favoring the CO 2 hydrogenation reaction through associative or dissociate mechanisms, as shown in Fig.10.54,55